1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for performing a power headroom reporting (PHR) procedure in a wireless communication system.
2. Related Art
Universal mobile telecommunications system (UMTS) is a 3rd generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). The long-term evolution (LTE) of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS.
The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3GPP LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement.
Transmit power needs to be properly regulated in order for a user equipment (UE) to transmit data to a base station (BS). When the transmit power is too low, the BS may not be able to correctly receive the data. When the transmit power is too high, even though the UE can receive data without any problem, it may act as an interference to another UE for receiving data. Therefore, the BS needs to optimize power used in uplink transmission of the UE from a system aspect.
In order for the BS to regulate the transmit power of the UE, essential information must be acquired from the UE. For this, power headroom reporting (PHR) of the UE is used. A power headroom implies power that can be further used in addition to the transmit power currently used by the UE. That is, the power headroom indicates a difference between maximum possible transmit power that can be used by the UE and the currently used transmit power. Upon receiving the PHR from the UE, the BS can determine transmit power used for uplink transmission of the UE at a next time on the basis of the received PHR. The determined transmit power of the UE can be indicated by using a size of a resource block (RB) and a modulation and coding scheme (MCS), and can be used when an uplink (UL) grant is allocated to the UE at a next time. Since radio resources may be wasted if the UE frequently transmits the PHR, the UE can define a PHR trigger condition and transmit the PHR only when the condition is satisfied.
According to a PHR trigger condition, there may be a case where the UE cannot transmit the PHR in a specific situation. In this case, a method of effectively determining a PHR trigger condition is required so that the UE can transmit the PHR.